This article is written by Amy from The Running Dietitian.
Carbohydrates are the key to running your best. While your body can technically use fat or protein for fuel, neither comes close to the efficiency of carbohydrates. They are your body's preferred and most effective energy source.
Despite this, carbohydrates have been unfairly caught in the crossfire of diet culture, leaving many runners confused or even discouraged about how much to eat. The matter of fact is, if you want to run well, carbohydrates are not the enemy.
The Science Behind Carbohydrates As Fuel (Made Simple)
When you eat carbohydrates, your body breaks them down into glucose, which is then stored in your muscles and liver in the form of glycogen. Glycogen is your body's primary and most readily available fuel source during exercise.
Think of it as your body's petrol tank. When you run, your muscles draw directly from those glycogen stores to sustain effort. Fat and protein can serve as secondary fuel sources, but they require significantly more time and energy to convert, making them far less efficient during moderate to high intensity running.
Running with depleted carbohydrate stores is the equivalent of attempting a long drive on an almost empty tank. You may manage for a time, but performance will eventually suffer. Understanding what carbohydrates actually do inside your body is the first step toward fueling with confidence.
The misconception that carbohydrates are harmful stems largely from diet culture, where they are often painted as the enemy. For runners, however, science tells a different story. Well stocked glycogen stores support sustained energy, mental clarity, and overall performance.
What Happens When You Don’t Eat Enough Carbs
Most runners have experienced a training run that felt inexplicably hard. Heavy legs, low motivation, a pace that should feel comfortable suddenly feeling harder than it should. Before assuming fitness has declined, it is worth considering if your carbohydrate intake may be the culprit. Under-fueling is one of the most common and overlooked reasons for poor performance.
When carbohydrate availability is low, the effects extend beyond feeling tired during a run. Over time, chronic under-fueling can impair your ability to hit target paces, weaken your immune system, and increase your susceptibility to injury and illness.
At the more serious end of the spectrum sits RED-S, Relative Energy Deficiency in Sport, a condition that affects both physical and hormonal health and carries long term consequences if left unaddressed.
It is also worth noting that total calorie intake does not tell the whole story. Even runners who are eating enough calories can still experience the effects of low carbohydrate availability if the timing or distribution of their carbohydrate intake is off.
Fueling and Weight Goals: They're Not Opposite
Questions about carbohydrates and body composition are common given how much conflicting information is out there.
Here is what the research consistently shows: chronic under-fueling, particularly low carbohydrate availability during regular training, does not reliably support fat loss.
In fact, it often works against it. When the body is consistently starved of adequate fuel, it adapts in ways that will not support training. It begins breaking down muscle tissue for energy, slowing your metabolic rate, and raising stress hormone levels. Over time, this also increases your risk of injury.
For runners looking to lose weight, the goal should not be simply to eat less. It is to eat in a way that protects your training, preserves and builds lean muscle, and still gives your body what it needs to recover. Fueling your performance and taking care of your body composition are not opposing goals if approached thoughtfully.
A Note on Individual Variation
Carbohydrate needs are not one size fits all. How much you need will vary depending on gender, genetics, training, and age, to name a few. Understanding this variation is just as important as understanding why carbohydrates are necessary for runners.
For women, hormonal fluctuations throughout the menstrual cycle can influence energy availability, appetite, and how the body utilizes fuel. Fasted training is another area where women, on average, tend to be more vulnerable to the negative effects of low carbohydrate availability.
For plant-based runners, the sources of carbohydrate will be very similar. Whole grains, legumes, fruits, starchy vegetables, and oats are all effective sources that can support training.
Across the board, one principle holds true for every runner regardless of gender, diet, or age. Carbohydrate needs scale with training load. The more you run, the more carbohydrates you will need to support training and recovery.
Learning to match your intake to your output, rather than applying a fixed daily number, is one of the most effective shifts you can make in your approach to fueling.
Takeaways
Carbohydrates are not the enemy of performance or body composition. They are the fuel that makes consistent training possible, and without adequate intake, your body is forced to adapt in ways that work against your goals.
Under-fueling may feel like discipline, but over time it impacts training and overall health in a negative way. Whether you are chasing a personal best or wanting to feel strong on every run, matching your carbohydrate intake to your training load is one of the most powerful things you can do for your running.